3GPP LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme. In a radio communication system to which 3GPP LTE is applied, a base station (which may also be called “eNB”) transmits a synchronization signal (Synchronization Channel: SCH) and broadcast signal (Broadcast Channel: BCH) using predetermined communication resources. A terminal (which may also be called “UE”) captures SCH and thereby secures synchronization with the base station. The terminal then reads BCH information and thereby acquires a parameter specific to the base station (e.g., frequency bandwidth) (see NPLs 1, 2 and 3).
After completion of the acquisition of the parameter specific to the base station, the terminal sends a connection request to the base station and thereby establishes communication with the base station. The base station transmits control information to the terminal with which communication has been established via a downlink control channel such as PDCCH (Physical Downlink Control Channel) as appropriate.
The terminal then performs “blind detection” of a plurality of pieces of control information (which may also be called “downlink control information (DCI)”) included in the received PDCCH signal. That is, the control information includes a CRC (Cyclic Redundancy Check) portion and this CRC portion is masked with a terminal ID of the transmission target terminal by the base station. Therefore, the terminal cannot determine whether or not the received control information is control information intended for the terminal until the terminal demasks the CRC portion with the terminal ID of the terminal itself. When the demasking result shows that CRC calculation is OK, it is determined in this blind detection that the control information is intended for the terminal itself. The downlink control information includes DL (downlink) assignment indicating assignment information of downlink data and UL (uplink) grant indicating assignment information of uplink data, for example.
Next, an uplink retransmission control method in 3GPP LTE will be described. In LTE, UL grant which is assignment information of uplink data is transmitted to the terminal by PDCCH. Here, in an FDD (Frequency Division Duplex) system, a UL grant indicates resource assignment within a target subframe which is the fourth subframe from the subframe in which the UL grant is transmitted.
Meanwhile, in a TDD (Time Division Duplex) system, a UL grant indicates resource assignment within a target subframe which is the fourth or after the fourth subframe from the subframe in which the UL grant is transmitted. This will be described more specifically using FIG. 1. In the TDD system, a downlink component carrier (which may also be called “downlink CC (Component Carrier)”) and an uplink component carrier (which may also be called “uplink CC”) are in the same frequency band, and the TDD system realizes downlink communication and uplink communication by switching between downlink and uplink in a time-division manner. For this reason, in the TDD system, a downlink component carrier can also be expressed as “downlink communication timing in a component carrier.” An uplink component carrier can also be expressed as “uplink communication timing in a component carrier.” Switching between the downlink component carrier and the uplink component carrier is performed based on a UL-DL configuration as shown in FIG. 1. The UL-DL configuration is indicated to the terminal by a broadcast signal called “SIB1 (System Information Block Type 1)” (SIB1 indication), the value thereof is the same throughout the entire system and the value is not expected to be changed frequently. In the UL-DL configuration shown in FIG. 1, timings in units of subframes (that is, units of 1 msec) are configured for downlink communication (DL: Downlink) and uplink communication (UL: Uplink) per frame (10 msec). The UL-DL configuration allows for building a communication system that can flexibly respond to requests for throughput for downlink communication and throughput for uplink communication by changing a subframe ratio between downlink communication and uplink communication. For example, FIG. 1 illustrates UL-DL configurations (Config#0 to 6) with different subframe ratios between downlink communication and uplink communication. In FIG. 1, a downlink communication subframe is represented by “D,” an uplink communication subframe is represented by “U” and a special subframe is represented by “S.” Here, the special subframe is a subframe when a downlink communication subframe is switched to an uplink communication subframe. In the special subframe, downlink data communication may also be performed as in the case of a downlink communication subframe. As shown by a solid line arrow in FIG. 1 (UL grant-PUSCH timing), a subframe to which uplink data for UL grant (PUSCH: Physical Uplink Shared Channel) is assigned is an uplink communication subframe which is the fourth or after the fourth subframe from the subframe in which the UL grant is indicated, and is uniquely defined as shown in FIG. 1.
Uplink retransmission control (UL retransmission control) supports non-adaptive retransmission in which retransmission data is assigned to the same resource as a resource to which uplink data is assigned at the time of the last transmission and adaptive retransmission in which retransmission data can be assigned to a resource different from a resource to which uplink data is assigned at the last transmission (e.g., see NPL 4). In non-adaptive retransmission, only PHICH (Physical Hybrid ARQ Indicator CHannel) for transmitting an ACK/NACK signal (response signal) in response to uplink data to the terminal is used as a channel for a retransmission control signal. When requesting the terminal to perform retransmission, the base station transmits a NACK to the terminal using PHICH and transmits an ACK using PHICH when not requesting the terminal to perform retransmission. In non-adaptive retransmission, since the base station can designate retransmission using only PHICH, non-adaptive retransmission has an advantage that the overhead of a control signal transmitted over downlink necessary to designate retransmission is small.
Here, in the FDD system, PHICH is indicated to the terminal using a resource within a target subframe which is the fourth subframe from the subframe in which uplink data is transmitted. Meanwhile, in the TDD system, PHICH is indicated to the terminal using a resource within a target subframe which is the fourth or after the fourth subframe from the subframe in which uplink data is transmitted. This will be described more specifically using FIG. 1. As shown by a broken line arrow (PUSCH-PHICH timing) in FIG. 1, a subframe to which ACK/NACK (PHICH) in response to uplink data (PUSCH) is assigned is a downlink communication subframe or special subframe 4 or more subframes after a subframe in which the uplink data is notified and is uniquely defined as shown in FIG. 1.
In adaptive retransmission, the base station transmits an ACK using PHICH while designating retransmission and a retransmission resource using UL grant for indicating resource assignment information. UL grant includes a bit called “NDI (New Data Indicator)” and this bit is binary having 0 or 1. The terminal compares an NDI of the received UL grant this time with an NDI of the last UL grant in the same retransmission process (HARQ (Hybrid ARQ) process), determines that new data has been assigned when there is a change in the NDI or determines that retransmission data has been assigned when there is no change in the NDI. Since adaptive retransmission allows the amount of resources and MCS (Modulation and Coding Scheme) to be changed according to a required SINR (Signal-to-Interference and Noise power Ratio) of retransmission data, adaptive retransmission has an advantage that frequency utilization efficiency improves.
Since a CRC (Cyclic Redundancy Check) is added to UL grant, a received signal with UL grant has higher reliability than PHICH. For this reason, when the terminal receives PHICH and UL grant, the terminal follows an instruction of UL grant.
FIG. 2 shows an example of a procedure for UL retransmission control in the terminal. In FIG. 2, in step (hereinafter abbreviated as “ST”) 11, the terminal determines whether or not there is UL grant. When there is UL grant (ST11: YES), the flow proceeds to ST12 and when there is no UL grant (ST11: NO), the flow proceeds to ST15.
In ST12, the terminal compares the NDI of UL grant this time with the NDI of the last UL grant in the same retransmission process and determines whether or not there is any change in the NDI. When there is a change in the NDI (ST12: YES), the flow proceeds to ST13 and when there is no change in the NDI (ST12: NO), the flow proceeds to ST14.
The terminal transmits new data to the base station in ST13 and transmits retransmission data to the base station through adaptive retransmission in ST14.
In ST15, the terminal determines whether or not PHICH is NACK. When PHICH is NACK (ST15: YES), the flow proceeds to ST16, and when PHICH is ACK (ST15: NO), the flow proceeds to ST17.
In ST16, the terminal transmits retransmission data to the base station through non-adaptive retransmission, and in ST17, suspending is applied, so that the terminal suspends retransmission control.
Next, a configuration of PHICH will be described.
It should be noted that in an LTE system and an LTE-A (LTE-Advanced) system which is an evolved version of LTE, one RB (Resource Block) is made up of 12 subcarriers×0.5 msec and a unit combining two RBs on the time domain is called “RB pair.” Therefore, the RB pair is made up of 12 subcarriers×1 msec. When the RB pair represents a block of 12 subcarriers on the frequency domain, the RB pair may be simply called “RB.” In addition, a unit of 1 subcarrier×1 OFDM symbol is called “1 RE (Resource Element).” 1 REG (Resource Element Group) is made up of 4 REs.
First, in coding of PHICH, ACK/NACK (1 bit) is subjected to three-time repetition. The number of PHICHs is one of {⅙, ½, 1, 2} times the number of RBs and is indicated by PBCH (Physical Broadcast Channel). The base station can transmit 8 PHICHs in 3 REGs (=12 REs) through code multiplexing and IQ multiplexing with SF (spreading factor)=4. The 8 PHICHs arranged on 3 REGs are called a PHICH group and expressed as “number of PHICH groups (that is, the number of resources) NgroupPHICH is 8.” In the FDD system, the number of PHICH groups NgroupPHICH takes the same value in all subframes.
Meanwhile, in the TDD system, as shown in FIG. 3A, a factor (mi) of number of PHICH groups is defined in each UL-DL configuration and each downlink communication subframe or special subframe. The total number of PHICH groups (=number of PHICH groups NgroupPHICH×factor mi of the number of PHICH groups) is changed for each subframe using this factor. In the FDD system, the factor of number of PHICH groups is always 1 irrespective of subframes.
The reason that the total number of PHICHs varies from one subframe to another in the TDD system will be described using FIG. 3B. FIG. 3B illustrates the number of subframes before a PHICH received by the terminal in subframe #n is associated with a PUSCH transmitted by the terminal. Blanks in FIG. 3B indicate that there are no PHICHs. For example, as shown in FIG. 3B, PHICH in subframe #1 of Config#0 is associated with PUSCH transmitted in subframe #7 which is 4 subframes earlier (see FIG. 1). In subframe #1 of Config#0, since PUSCH in one subframe is associated with PHICH in one subframe, factor mi of the number of PHICH groups is assumed to be 1 as in the case of the FDD system (see FIG. 3A). On the other hand, as shown in FIG. 3B, PHICH in subframe #0 of Config#0 is associated with PUSCHs transmitted in subframe #3 which is 7 subframes earlier and in subframe #4 which is 6 subframes earlier respectively. That is, in subframe #0 of Config#0, the terminal receives PHICHs corresponding to two PUSCHs. Thus, in subframe #0 of Config#0, twice as many resources for PHICH (hereinafter referred to as “PHICH resources”) as those in subframe #1 of Config#0 are required, and therefore factor mi of the number of PHICH groups is considered to be 2 (see FIG. 3A).
In FIG. 3B, two PHICHs intended for the same terminal received in the same subframe (e.g., subframes #0 and 5) are distinguished by parameter IPHICH. For example, in subframe #0 of Config#0, PHICH corresponding to PUSCH 7 subframes earlier corresponds to IPHICH=0 and PHICH corresponding to PUSCH 6 subframes earlier corresponds to IPHICH=1. The same applies to subframe #5 of Config#0. For PHICHs in other UL-DL configurations and subframes, IPHICH is always 0.
A PHICH resource is represented by a combination {ngroupPHICH, nseqPHICH} of an index of the total number of PHICH resources ngroupPHICH and an index of orthogonal sequence nseqPHICH. The index of the total number of PHICH resources ngroupPHICH and the index of orthogonal sequence nseqPHICH are expressed by following equations 1 and 2 respectively.(Equation 1)nPHICHgroup=(IPRB_RA+nDMRS)modNPHICHgroup+IPHICHNPHICHgroup  [1](Equation 2)nPHICHseq=(└IPRB_RA/NPHICHgroup┘+nDMRS)mod2NSFPHICH  [2]
Here, NPHICHSF is a spreading factor (SF) that varies depending on the length of a CP (Cyclic Prefix). IPRB_RA is a minimum value of a PRB (Physical RB) index to which PUSCH corresponding to PHICH is assigned. Meanwhile, nDMRS is a cyclic shift value of DMRS (Demodulation Reference Signal) included in UL grant that indicates PUSCH corresponding to PHICH. Since IPRB_RA and nDMRS depend on assignment of UL grant and PUSCH, a PHICH resource can be said to be implicitly indicated (implicit signaling) based on the assignment of UL grant and PUSCH. The determined PHICH resource is divided for every value of IPHICH. For example, in subframe #0 of Config#0, PHICH corresponding to PUSCH 7 subframes earlier and PHICH corresponding to PUSCH 6 subframes earlier are designed such that the PHICH resources do not conflict with each other.
Mapping of PHICH depends on a cell ID. Therefore, it is difficult to control interference of PHICH with other cells and PHICH may interfere with PDCCH and/or CRS (Cell-specific Reference Signal) in other cells. All of 3 REGs making up PHICH may be arranged on OFDM symbol #0 (not shown) or 3 REGs may be arranged one for each of OFDM symbols #0, #1 and #2 as shown in FIG. 4. Information indicating which PHICH arrangement is used is indicated to the terminal using a broadcast signal.
The number of OFDM symbols (1 to 3) occupied by PDCCH is determined based on the value of CFI (Control Format Indicator) indicated by PCFICH (Physical Control Format Indicator Channel) arranged on OFDM symbol #0. Moreover, when detecting PDCCH, the terminal performs blind detection on some resources in resource regions except resources occupied by PCFICH, PHICH and reference signals (hereinafter may also be referred to as “PDCCH resources”) of resource regions corresponding to the number of OFDM symbols indicated by CFI from OFDM symbol #0.
In the LTE-A system, studies are being carried out on changing UL-DL configuration (hereinafter referred to as “TDD eIMTA (enhancement for DL-UL Interference Management and Traffic Adaptation),” which may also be referred to as “dynamic TDD” or “flexible TDD”). Exemplary purposes of TDD eIMTA include provision of a service that meets the needs of users by flexible changes of a UL/DL ratio or reduction in power consumption at a base station by increasing the UL ratio in a time zone when traffic load is low. As a method of changing UL-DL configuration, the following methods are under study in accordance with the purpose of change: (1) method using indication of an SI (System Information) signaling base, (2) method using indication of an RRC (higher layer) signaling base, (3) method using indication of a MAC (Media Access Control layer) signaling base and (4) method using indication of an L1 (Physical Layer) signaling base.
Method (1) is to change the least frequent UL-DL configuration. Method (1) is suitable for a case where the purpose is to reduce power consumption at a base station by increasing the UL ratio, for example, in a time zone when traffic load is low (e.g., midnight or early morning). Method (4) is to change the most frequent UL-DL configuration change. The number of terminals connected is smaller in a small cell such as a pico cell than in a large cell such as a macro cell. In a pico cell, UL/DL traffic in the entire pico cell is determined depending on the level of UL/DL traffic in a small number of terminals connected to the pico cell. For this reason, UL/DL traffic in the pico cell fluctuates drastically with time. Thus, method (4) is suitable for a case where UL-DL configuration is changed to follow a time fluctuation of UL/DL traffic in a small cell such as a pico cell. Method (2) and method (3) are positioned between method (1) and method (4) and suitable for a case where UL-DL configuration is changed with medium frequency.